Whenever a number of signals are available for reception to the MR as well as to the RR, it is possible to locate the MR. Examples of radio signals that exist with worldwide coverage are satellite signals such as Global Positioning System (GPS), Iridium, Globalstar, etc. Examples of radio signals that can penetrate structures such as buildings are land-based systems such as cellular, PCS, paging, and ESMR, among others. Both satellite-based signals and terrestrial-based signals can be used to locate the MR by relying on their reception at both the MR and the RR. In this patent, we refer to such a location system as a handset-based Wireless Location System (WLS) (or Forward/downlink-based WLS), as opposed to a network-based WLS (or Reverse/uplink-based WLS), which locates a Mobile Transmitter (MT) using a plurality of receivers that receive the signal transmitted from the MT. An advantage of a network-based WLS is that it can be passive (in that it can locate MR's without modification or addition to wireless transmissions) and that no modifications to the hardware or operation of the MT are required for the WLS to locate the MT. On the other hand, a handset-based WLS has the advantage of requiring no modification of existing network infrastructure. The location in the handset-based WLS can be performed either at the MR, or at the RR, or at a Third Receiver (TR). When the location is performed at the MR, the RR is required to transmit the Reference Information to the MR. When the location is performed at the RR, the MR is required to transmit the Location Information to the RR. When the location is performed at the TR, both the MR and the RR are required to transmit their corresponding information to the TR.
Radio signals can interfere with one another unless multiple access techniques are employed. Existing radio signals use any one of a number of multiple access techniques such as: Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA). CDMA is used in IS-95, the North American standard for CDMA CTs, as well as in GPS. FDMA is used in AMPs, the North American standard for analogue CTs. TDMA is used in IS-136, the North American standard for TDMA CTs, and in GSM, the European standard for TDMA CTs. Without loss of generality, we will use in this patent GPS signals as an example of a satellite-based radio signal and IS-95 signals as an example of a land-based radio signal. This choice does not preclude any other standard or any other multiple access technique. In this patent, we refer to a transmitter as a Base Station Transmitter (BST). It is important to note that a RR is not required in this patent if the BSTs are referenced to the same reference signal.
The need for wireless location finding and tracking of MRs is ever increasing. Some of the possible services for locating MRs are:
1. Enhanced Emergency Services: In recent years pressure has been mounting for the development of technology to position Cellular Telephones (CTs). The primary driving force has been enhanced 911 (E911) services for wireless telephone subscribers. E911 services provide the 911 operator with information such as calling number, street address, and the primary subscriber's name. A ruling concerning E911 emergency calling systems was released by the FCC in October, 1996 (CC Docket No. 94-102) and clarified in December 1997. In the document, the FCC requires that by October 2001, a wireless 911 caller be located with a horizontal accuracy of 125 meters RMS (Root Mean Square). PA0 2. Tracking of Fraudulent Calls. PA0 3. Tracking of Stolen Vehicles: Conventionally, to track stolen vehicles requires installing an RF tag such as in Bird, U.S. Pat. No. 5,418,537 issued May, 23, 1995, leaving it permanently on in each vehicle to be tracked and a new infrastructure for the BSs throughout the desired service area. An alternative solution is to either use existing CTs to take advantage of the existing cellular coverage and the broad availability of inexpensive CTs, or to use GPS receivers to take advantage of the availability and accuracy of satellite signals. PA0 4. Fleet Management for Courier and Transportation Businesses: Once again, to manage a fleet requires installing an RF tag such as in Song, U.S. Pat. No. 5,208,756 issued May, 4, 1993, and Sheffer et al., U.S. Pat. No. 5,218,367 issued Jun. 8, 1993. Instead, existing CTs can be used to take advantage of the existing cellular coverage and the broad availability of inexpensive CTs. Alternatively, GPS receivers can be used to take advantage of the availability and accuracy of satellite signals. PA0 5. Location-sensitive billing.
Without loss of generality, we will assume throughout this document that the intention of the handset-based WLS is to estimate the horizontal position of the MR as well as its horizontal velocity. In case both the vertical and the horizontal position of a MR are to be estimated, an extra independent measurement is required in addition to the minimum number required for horizontal positioning.